1. Technical Field
This invention relates to a transmitter for optically transmitting analog electric signals and to a digital transmission system.
2. Discussion of Related Art
The analog electric signals are, for example, television signals or video signals. An optical transmission system for the optical digital transmission of television signals is known from an article entitled "DIAMANT"--Ein digitales optisches Fernsehverteilnetz", telecom praxis 2/1993, pages 38 to 45. The television signals are converted from analog to digital form, generally by pulse code modulation, electrically multiplexed, and converted from electrical to optical form; an optical pulse commonly corresponds to an electric pulse, and thus to one bit. The optical digital television signals are transmitted through a passive optical transmission network consisting of optical fibers and optical splitters to a plurality of receivers. Each receiver contains an optical-to-electrical transducer and a digital-to-analog converter for recovering the analog electric signals. The number of multiplexed television signals is limited by the resolution and speed of the available digital-to-analog converters. Each digital-to-analog converter is an active device which needs to be controlled and synchronized.
In each receiver, the digital-to-analog conversion must be followed by a frequency translation in order to transfer the television signals from the intermediate-frequency band to the UHF band.